press f12 or ESC button of keyboard
goto ghrub section select. 2 option i.e kermal press.E for edit the 2 option dom t do any thing just press space button and add value i.e 1 then goto back to the ghrub press B then system will boot wait for min
then enter a command passwd anpress enter it will ask enter new unix password and confir the unix passwd then it will show like this "your passwd has successly updated after that write a command